Ammonium nitrate propellant composition providing exhaust gases of reduced temperature



United States Patent Ofifice 3,161,550 Patented Dec. 15 1964 This invention relates to ammonium nitrate compositions particularly suitable for use as gas generating compositions, which compositions are characterized by relatively low exhaust gas temperatures.

Ammonium nitrate explosive compositions comprise the ammonium nitrate oxidizer, a catalyst for accelerating the decomposition of the ammonium nitrate and an oxidizable material which utilizes the excess free oxygen produced in the decomposition of the ammonium nitrate. These oxidizable materials commonly have included virtually all classes of compounds and elements which can react with oxygen. Usually, these oxidizable materials are organic compounds or polymers or mixtures of organic compounds which function as a matrix former or binder by holding the, normally, finely divided ammonium nitrate particles and catalyst into a predetermined stable configuration. The stable configuration may be of particular shapes such as cylinders, tubes, discs and cubes; also, they may be irregular shapes obtained by crushing larger blocks to obtain a spectrum of particle size. The configuration must have dimensional stability over the range of ordinary atmospheric temperatures for the usual storage times required by military service.

For explosive purposes, or rocket propulsion purposes, the temperature of the combustion gases is not critical and usually higher temperatures are desired since they provide higher volume and/or higher pressure product. Recently many uses have been made of ammonium nitrate type explosive compositions as sources of combustion gases at elevated pressure for the purpose of driving turbines which in turn provide auxiliary power for uses in rocketry, or other high demand, limited time requirements. For this gas generation utility, the exhaust gases from the combustion chamber must be passed through conduits to the point of ultimate use. The gas generation period may extend from a very few seconds to as much as severaFminutes. In the more extended time durations, it is evident that the conduits carrying the exhaust gases can become heated to high temperatures such that significant reductions in metal strengths take place with a consequent requirement for either more metal present, or the use of expensive alloys. The desire for light weight gas generator installations, and at the same time a minimum of capital cost, has led to a search for ammonium nitrate explosive compositions providing exhaust gas of much reduced temperatures over those presently obtained. The principal object of this invention is an ammonium nitrate explosive composition suitable for gas generating purposes which does provide exhaust gases of markedly lower temperature than provided heretofore by known compositions. Other objects will become apparent in the course of the detailed description of the invention.

The gas generator composition of the invention is a solid made up of cellulose acetate, acetyl triethyl citrate and triethyl citrate plasticizers for the cellulose acetate, alkali metal barbiturate catalyst for accelerating the decomposition of the ammonium nitrate, finely divided carbon burning rate promoter, aromatic hydrocarbon amine gassing inhibitor for promoting storage stability, and ammonium nitrate. These named ingredients are present in certain herein defined proportions expressed as weight percent of the total composition. Broadly, the solid composition of the invention contains about 8.'5-12.0% of cellulose acetate, about 9.0-111.5% of acetyl triethyl citrate, about 9.5-11.0% of triethyl citrate, about 1-3% of alkali metal barbiturate, about 23% of finely divided carbon, about 1% of aromatic hydrocarbon amine gassing inhibitor, and the remainder essentially ammonium nitrate. Frequently a composition will include minor amounts of other ingredients which are useful for some particular purpose, for example, surfactants to improve the blending characteristic of the mixture; phenyl morpholine stabilizers, oximes to improve ignitability; etc.

An outstandingly good composition with respect to exhaust gas temperature and absence of solid particles in the gas stream consists of cellulose acetate, about 11.2%; acetyl triethyl citrate, about 11.1% triethyl citrate, about 10.4%; carbon black, about 3.0%; sodium barbiturate, about 1.0%; toluene diamine, about 10%; and ammonium nitrate, about 62.3

Another fine quality composition with respect to exhaust gas cleanliness and relatively low exhaust gas temperature consists of cellulose acetate, about 8.9%; acetyl triethyl citrate, about 9.0% triethyl citrate, about 10.0%; carbon black, about 3.0%; sodium barbiturate, about 1.0%; toluene diamine, about 1.0%; and ammonium nitrate, about 67.1%.

The ammonium nitrate is the predominant component in the composition and is normally present in an amount of about 59-70%. The ammonium nitrate content is dependent upon the other components present as well as the desired oxygen balance of the composition. In order to obtain a soot-free exhaust gas (non-smoky), it is desirable to have a composition which is at least theoretically in oxygen balance between the free oxygen produced from the ammonium nitrate and the oxygen demand of the other components.

The ammonium nitrate may be the high purity material commonly produced by synthetic plants today, or it may be technical grade containing small amounts of inorganic impurities. In addition to the ammonium nitrate, for special purposes, sodium nitrate or potassium nitrate may be present in an appreciable amount. The decomposition rate of the ammonium nitrate is influenced by the particle size. For gas generation purposes the ammonium nitrate is finely divided. Particularly suit-able ammonium nitrate will contain about weight percent of material having a screen size greater than 80 mesh and smaller than 30 mesh. The more finely powdered ammonium nitrate is used where higher burning rates are desired.

In this composition, the catalyst for accelerating the decomposition of the ammonium nitrate is alkali metal barbiturate, preferably the mono-salt of the acid. Sodium barbiturate is a preferred catalyst. The catalyst is present in an amount of about l3%. The amount of catalyst present has a bearing on the burning rate, expressed as inches per second at a given temperature and pressure, with the burning rate increasing as the catalyst amount increases.

Finely divided carbon is present as a burning rate promoter in an amount of about 2-3%. It is possibleto control the burning rate of the final composition by balancing the amount of the catalyst and the amount of carbon present. Although any finely divided form of carbon or graphite may be used, it is preferred to utilize the carbon blacks.

Ammonium nitrate compositions in general tend to develop gas in storage at elevated temperatures. The composition of the invention includes a gassing inhibitor component. Aromatic amines when introduced into the ammonium nitrate based grain containing catalyst and finely divided carbon have the Very desirable characteristic of decreasing the amount of gassing in high temperature storage and frequently even eliminating or essentially eliminating gassing for prolonged periods of time. The aromatic amine gassing inhibitors of this invention are illustrated by diphenylamine, dinaphthylamine and phenyl naphthylamine usually having the linkage between the naphthyl radical and the nitrogen either in the alpha or beta position, and those amines represented by the empirical formula:

In this empirical formula Z is an aromatic nucleus selected from the class consisting of phenyl and naphthyl; R is selected from the class consisting of hydrogen and alkyl containing from 1 to 12 carbon atoms; R and R" are selected from the class consisting of hydrogen, and alkyl containing from 1 to 4 carbon atoms; and x is an integer from 1 to 3.

Examples of monoamino-containing compounds are: aniline (monophenylamine, monoarnino benzene), 1- naphthylamine, toluidine (methyl aniline) xylidine (dimethylaniline), dodecyl aniline, N-methyl aniline, N,N- dimethyl aniline, N-sec-butyl aniline.

Examples of the diamino compounds are: diamino benzene (phenylene diamino), diamino toluene (toluene diamine), diamino naphthylene, methyl diamino naphthylene, dodecyl diamino naphthyle ne, N sec-butyl diamino benzene, N,N-di sec-butyl diamino benzene, and N-methyl diamino naphthylene.

Examples of triamino compounds are: triamino benzene, triamino naphthylcne, triamino toluene, and triarnino methyl naphthylene.

In general, these aromatic hydrocarbon amine gassing inhibitors are present in an amount of about 1%; commonly at least about 0.5% and not more than 1.5% will be present.

The material which forms the matrix wherein the ammonium nitrate and other solid components are dispersed is also known as the binder material. Herein the binder material consists of a thermoplastic material formed from cellulose acetate and a combination of the plasticizers, acetyl triethyl citrate and triethyl citrate.

Both acetyl triethyl citrate and triethyl citrate are well known plasticizers for cellulose acetate. Either the pure compounds or the commercial technical grade may be used as plasticizers for the cellulose acetate in the composition of the invention. Cellulose acetate is used to provide strength to the thermoplastic binder. Any of the cellulose acetates known to commerce which are plasticizable to the required degree by the defined proportions of the defined plasticizers may be used. Particularly suitable are those cellulose acetates which have a combined acetic acid content in the range of 5058%. A particularly suitable cellulose acetate is made by Eastman Chemical Products, Inc., and sold as Eastman-60. In some compositions, it is necessary to use a mixture of two or more cellulose acetates in order to obtain the desired binder properties. An outstanding cellulose acetate for use in admixture with Eastman-60 is Eastman-200.-

The relative proportions of the three named binder components will be determined by the exhaust gas temperature desired, and the physical characteristics of the composition, for example, the ability of the composition to retain its ordinary temperature configuration when exposed to elevated temperatures is determined, in large part, by the cellulose acetate component by both the type and amount present. It has been found that compositions producing relatively low exhaust gas temperatures, clean exhaust gases and meeting military specifications with respect to configuration stability, chemical stability as measured by gas evolution and resistance to cracking can be made by the use of binders having proportions as set forth hereinbefore.

Illustrations Three gas generating compositions were prepared by introducing the organic binder materials into a heated ettle provided with a mechanical stirrer. The binder materials were raised to a temperature where they became a viscous homogeneous liquid. The catalyst and other additives were blended into the molten binder before the addition of the solid ammonium nitrate. During the blending of the ammonium nitrate, the temperature of the mixing operation was held below 130 C. In this instance, the ammonium nitrate was of a screen size: passing through a 14 number screen and retained by a 30 number screen; about 12% retained by an 80 number screen and passing through a 30 number screen; about 5% smaller than 80 number screen and the remainder larger than 14 number screen.

Two of the three compositions prepared fall within the scope of the invention; one composition was prepared to illustrate a typical conventional gas generator composition now used for military purposes. The components and weight percentages of each present in each composition are set out in tabular form below. The compositions illustrative of the invention are identified as Mark 4351 and Mark 4192; the typical composition now used is identified as Mark 3157.

Data were obtained on burning rates at 70 F. and 1,000 p.s.i.a. by the Crawford Bomb Method. The'approximate gas composition produced was obtained and the exhaust gas temperature calculatedattempts to measure exhaust gas temperature were subject to such lack of reproducibility as to be valueless for evaluation of differences between compositions. The smokiness of the gas stream was noted. All of the compositions here passed the military specification requirements.

Composition Mark Mark Mark Ammonium nitrate 62. 3 67. 1 62. 2

Cellulose acetate (Eastman-60L 11.2 6. 2 10. 4

Cellulose acetate (Eastman-200) None 2. 7 None Aeetyl triethyl citrate. 11.1 9. 0 11. 9

'Iiricthyl citrate 10.4 10.0 None Dinitrophenoxyethanol None None 10. 4

Carbon black 3.0 3.0 3.1

Sodium barbiturate 1.0 1. 0 1. 0

Toluene diamino 1.0 1.0 1.0

Exhaust gas temperature, F 1, 275 1, 700 1, 800

Burning rate, inches per second 0. 049 0. 049 0.053

Smoke None None None Gas Composition, Mol percent:

These data establish the fact that the change in binder composition has resulted in an unobvious, very large change in exhaust gas temperature in the gas of composition Mark 4351. The composition Mark 4192 shows the need for control of binder composition in order to obtain a particular desired temperature.

Thus having described the invention, what is claimed is:

1. A solid composition consisting of cellulose acetate, about 8.512.0% acetyl triethyl citrate, about 9.011.5%; triethyl citrate, about 9.51l.0%; alkali metal barbiturate, about l3%; finely divided carbon, about 23%; aromatic hydrocarbon amine gassing inhibitor, about 1%; and the remainder of said composition essentially ammonium nitrate.

2. The composition of claim 1 wherein said gassing inhibitor is toluene diamino.

3. The composition of claim 1 wherein said barbiturate is sodium barbiturate.

4. A solid, gas generator composition consisting of cellulose acetate, about 11.2%; acetyl triethyl citrate, about 11.1% triethyl citrate, about 10.4%; carbon black, about 3.0%; sodium barbiturate, about 1.0% toluene diamine, about 1.0%; and ammonium nitrate, about diamine, about 1.0%; and ammonium nitrate, about 62.3%. 67.1%.

5. A solid, gas generator composition consisting of cellulose acetate, about 8.9%; acetyl triethyl citrate, Refaencesghed m the file of thls patent about 9.0%; triethyl citrate, about 10.0%; carbon black, 5 UNITED STATES PATENTS about 3.0%; sodium barbiturate, about 1.0%; toluene 3,067,076 Butcher et a1 Dec. 4, 1962 

1. A SOLID COMPOSITION CONSISTIG OF CELLULOSE ACETATE, ABOUT 8.5-12.0%; ACETYL TRIETHYL CITRATE, ABOUT 9.0-11.5%; TRIETHYL CITRATE, ABOUT 9.5-11.0%; ALKALI METAL BARBITURATE, ABOUT 1-3%; FINELY DIVIDED CARBON, ABOUT 2-3%; AROMATIC HYDROCARBON AMINE GASSING INHIBITOR, ABOUT 1%; AND THE REMAINDER OF SAID COMPOSITION ESSENTIALLY AMMONIUM NITRATE. 